In the wild, they can live to 9-10 years. It's good that you got to see him, he's quite old and probably won't be around much longer. I look at the white on the nose and the chest to gauge age, along with the general condition of the body.
This wee beasty is swaybacked, has a pot belly with a good amount of white up front. I'd say he was at the top end of the age scale.
I killed this California Blacktail in 2016. We spotted him sitting in the middle of an open prairie. He appeared to be a very old deer way past his prime, just taking advantage of the warm sun during his last days. We agreed that he was probably nearing the end and would probably not survive the winter. After some contemplation, I decided this was the right deer to take. At any rate, it would be a kinder death (if there is such a thing) for the old gentleman. Rather than being eaten alive by a mountain lion or ripped to pieces by a pack of coyotes. I ranged him at 166 yards, and the 30/06 Encore pistol delivered the 150 gr, Ballistic Tip on its mission of mercy. The old deer died in his tracks. His live weight was 183 pounds, and he appeared to be in good health. Even though he was a brute in body size, his headgear was only 2 x 3. Closer examination showed that he was in regression. From the antler configuration we surmised that he was probably at least a 4 x 4 in his prime. Any deer that weighs 80% as much as I do is a pretty good deer!
That's a trophy for sure. How could you tell he was in regression from the horns? I ask because I shot a 4x5 a couple of years ago, that I'd been chasing for 4 years. No body believed me when I said he was bigger the first year I spotted him.
The heaviest deer I've personally seen weighed 175 lbs. It was taken at 55 MPH with a jeep Cherokee, neither the deer nor the Cherokee survived the encounter. Strangely enough, the driver was a game warden.
Look closely at the antlers, and you will see bumps where there were previously tines. The all white face is a testament to his age, as is the live weight of 183 pounds (actually weighed. This is the second largest Blacktail that I have killed. I killed one in 2018 that tipped the scales at 197 pounds, which is my largest Blacktail to date. This was definitely not a young deer.
I killed a 9 year old 4x3 off of our property near Coalinga in 2010. Very white in the muzzle and could have used dentures, he dressed out at 158 pounds. I would have loved to have seen what he looked like in his absolute prime. He was still impressive but you could tell he sported a couple more points at some point.
The face on the buck in the first picture looks incredibly similar.